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Measuring distance between stars and planets
Measuring distance between stars and planets

Measuring distances in space

Space is very vast, even our solar system is so vast that to explore it with our current technology would take decades and a lot of resources. If we use the normal units to measure the distance between planets and the star, we will have to write thousands and millions of kilometres each time we need it. So, the astronomers defined new sets of units to measure distance of the heavenly bodies. They are all mentioned below.

Lunar Distance (LD)

The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the centre of Earth to the centre of the Moon. The LD is a unit of measure in astronomy. More technically, it is the semi major axis of the geocentric lunar orbit.

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The lunar distance is approximately 384,400 km. this is roughly 30 times earth’s diameter. The actual distance varies over the course of the orbit of the Moon, from 356,500 km at the perigee to 406,700 km at apogee, resulting in a differential range of 50,200 km.

Lunar distance is commonly used to express the distance to near-Earth object encounters. Lunar semi-major axis is an important astronomical datum; the few millimetres precision of the range measurements determines semi-major axis to a few decimetres; it has implications for testing gravitational theories such as general relativity, and for refining other astronomical values such as Earth mass, Earth radius, and Earth's rotation. The measurement is also useful in characterizing the lunar radius, the mass of the Sun and the distance to the Sun.

Astronomical unit (AU)

It is roughly the distance between earth and the Sun and equal to about 150 million kilometres. The actual distance varies by about 3 percent as earth orbits the sun from a maximum (aphelion) to a minimum (perihelion) and back again once year. The astronomical unit was originally conceived as the average of Earth's aphelion and perihelion; however, since 2012 it has been defined as exactly 149597870700 m.

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The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,000 astronomical units (au), i.e., 30.9 trillion kilometres. Parsec is obtained using parallax and trigonometry and is defined as the distance at which 1 au subtends an angle of one arcsecond.  


Description automatically generated                    The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun. The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to make calculations of astronomical distances from only raw observational data easy for astronomers. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage.

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           One could fit all the other planets in the solar system between earth and the moon

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                                  The distance of various planets from the Sun

Light Year (LY)

The light-year, alternatively spelled lightyear, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometres (9.46×1012 km). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days).Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry.

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Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.


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                                The distance between star clusters and galaxy



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Facts about the largest planet of our solar system (Jupiter)
Facts about the largest planet of our solar system (Jupiter)

                                    The biggest planet in our solar system

Jupiter is one of the brightest planets in our skies and the largest and most massive planet in the Solar System. It has faint rings, numerous moons, and an unstable surface. Jupiter is considered the giant or the Jovian planet, together with Saturn, Uranus, and Neptune. When ancient astronomers named Jupiter after the Roman ruler of all gods, they had no idea about its enormous size surpassing other planets. Yet, they came up with a very fitting name.

Jupiter's size

With a radius of 69,911 km, Jupiter is the biggest planet in the Solar System. In comparison, the second-biggest Saturn has a radius of 58,232 km (36,184 mi). Jupiter is also the most massive planet — it’s more than twice as massive as all the other planets combined

Jupiter's orbit and rotation

Each planet takes a certain amount of time to complete one orbit around the Sun and one rotation around its axis. As we live on the Earth, we take the local days (24 hours) and years (365.25 days) as a standard. Let's see how different from our planet Jupiter is.

Despite being the largest planet, Jupiter is also the fastest spinning planet in the Solar System; therefore, it has the shortest days. One day on Jupiter takes slightly less than 10 hours — the exact time varies from 9 hours and 56 minutes around the poles to 9 hours and 50 minutes close to the equator. The reason behind this difference is that Jupiter is a gas planet and doesn’t rotate as a solid sphere. Instead, its equator rotates slightly faster than the polar regions, which leads to the distinction in the day length in different areas.

One Jovian year takes 11.8618 Earth years or 4,332.59 Earth days. In comparison, the second-largest planet Saturn has an orbital period of around 29 Earth years and the smallest Mercury revolves around the Sun every 88 Earth days

How many Earths can fit in Jupiter?

It would take more than 1,300 Earths to build a single Jupiter. If the gas giant were the size of a basketball, the Earth would be the size of a grape.

How far is Jupiter from the Sun?

The gas giant is 5.2 AU from the Sun or 778 million km away. In comparison, Mercury, the closest planet to the Sun, is 0.4 AU or roughly 58 million km away from our star. (One astronomical unit (AU) is the distance between the Sun and the Earth.)

How far is Jupiter from the Earth?

The distance between planets is constantly changing because they are moving along their orbits. Jupiter is only 588 million km away when it’s closest to our planet and 968 million km at its farthest.

How long does it take to get to Jupiter?

For a simple flyby, it will take about 550-650 days as it happened with the Voyager spacecraft: Voyager 1 took only 546 days, and Voyager 2 took 688 days. However, if you’re planning to go into Jupiter’s orbit, you’ll need to be going slowly enough when you reach the planet. For example, NASA’s Galileo spacecraft flight duration was 2,242 days before it finally arrived at Jupiter.

What is Jupiter made of?

Jupiter doesn’t have a solid surface; its atmosphere just gets denser the farther down you go, transitioning into a liquid layer surrounding a small core. Simply, it means that the atmosphere of Jupiter makes up almost the entire planet. Jupiter (and its atmosphere) consists of about 90 % hydrogen and 10 % helium — which is very similar to the Sun’s composition.

Jupiter’s formation

Like other planets in the Solar System, Jupiter formed about 4.5 billion years ago, when gravity pulled gas and dust together to create the gas giant. The planet took most of the mass left over after the formation of the Sun and became more than twice the combined material of the other bodies in the Solar System. About 4 billion years ago, Jupiter settled into its current position as the fifth planet from the Sun.

Jupiter's structure

We still don’t know for sure what Jupiter’s core looks like. It might consist of solid materials or be a thick, boiling, dense soup. What we know is that the core is surrounded by a layer of liquid metallic hydrogen that extends out to 90% of the planet’s diameter.

Jupiter's surface

This gas giant doesn’t have the hard surface as we do on the Earth. The planet is mostly swirling gases and liquids. A spacecraft can’t land on it or fly through the planet due to the extreme pressures and temperatures that will crush, melt, and vaporize it.

What is the Great Red spot on Jupiter?

The Great Red Spot is a giant storm about twice as wide as the Earth located in Jupiter’s Southern Hemisphere. It consists of crimson-coloured clouds that spin counter-clockwise at a speed that exceeds any storm’s speed on the Earth.

This storm was first observed in 1878; however, Gian Domenico Cassini in 1665 mentioned “Permanent Storm,” which is believed to be the Great Red Spot. Such a long-lasting storm can be explained by the absence of a solid surface on Jupiter. On the Earth, hurricanes disintegrate when they reach solid ground, but the Red Spot simply doesn’t have land to collide with.

However, the Great Red Spot has been shrinking over the years: from a length of about 40,000 km (24,850 mi) in 1879 to nearly 15,000 km (9,320 mi) in 2021. The reasons behind it are unknown.

Jupiter's moons

Jupiter and its numerous satellites resemble a miniature Solar System and present a scientific interest for astronomers around the world.

How many moons does Jupiter have?

Jupiter has 79 moons: 53 of them are named, and 26 are waiting for an official name. Most of them are small — about 60 satellites are less than 10 km (6.2 mi) in diameter. The number of moons is constantly changing; in 2003, astronomers discovered 23 new moons, then, in 2018, 12 more Jovian moons were found. As of 2021, Jupiter is losing to Saturn on the number of satellites; according to NASA, the ringed planet has 82 moons.

What is Jupiter's 4 largest moons?

Jupiter’s four largest moons are Io, Europa, Ganymede, and Calisto. They’re called the Galilean satellites after their discoverer and are as remarkable as Jupiter itself.

The largest one, Ganymede, is bigger than Mercury and is known as the most gigantic satellite in the Solar System. It even has its own magnetic field! Europa, in its turn, has a very high potential to be habitable — there is evidence of a vast ocean just beneath its icy surface. It’s thought to have twice as much water as the Earth. Io is the most volcanically active body in the Solar System, with hundreds of volcanoes on it.

Calisto, which is about the same size as Mercury (99% of its diameter, to be precise), is the third-largest satellite in our Solar System and may look boring against the background of the other three moons. However, in the 1990s, NASA’s Galileo spacecraft revealed that there might be a salty ocean beneath Calisto’s surface.

Jupiter's rings

The Jovian ring system was the third ring system discovered in the Solar System, after those of Saturn and Uranus. Jupiter’s rings are faint and mostly consist of dust; they’re likely leftovers from meteor bombardment of Jovian moons.

How many rings does Jupiter have?

Jupiter has four rings: the closest to the planet faint halo ring, a relatively bright but very thin main ring, and two wide and thick gossamer rings — the Amalthea and the Thebe. The last two are named after the moons of whose material they consist of.

Are Jupiter's rings visible?

We surely won’t see the Jupiter rings with the naked eye since they’re too faint and tenuous. For ground-based observation, the largest telescopes available are required. Even from space, they’re visible only when viewed from behind Jupiter and are lit by the Sun or directly viewed in the infrared.

Missions to Jupiter

Since 1973, nine spacecraft have visited Jupiter. The most noteworthy ones are:

  1. The first one was NASA’s Pioneer 10 that provided hundreds of Jupiter’s photos and collected some measurements. The Pioneer 11 in 1974 got three times closer to the planet than its predecessor.
  2. In 1979, the famous Voyager spacecraft discovered the Jovian ring system and took thousands of pictures of clouds and storms on the planet. Those pictures also showed that the mysterious Great Red Spot is a gigantic storm. Moreover, Voyager 1 and 2 discovered dozens of volcanoes on Jupiter’s moon Io — the first found active volcanoes on another space object.
  3. NASA’s Galileo probe became the first spacecraft to enter Jupiter’s orbit; it arrived on the planet in 1995. The Galileo mission, among many other things, examined Jupiter’s atmosphere and immense magnetic field and closely studied the Galilean moons. Several years later, in 2000, the Cassini spacecraft that was heading to Saturn took some of the best photos we have of Jupiter.
  4. The second spacecraft ever to enter Jupiter’s orbit is called Juno. It arrived at Jupiter in 2016 and will be exploring the gas giant until September 2025 or the spacecraft's end of life.


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Light is an electromagnetic radiation which is visible by the human eye, within the spectrum of the electromagnetic spectrum. Visible light has wavelength range between 400 to 700 nm (nanometres), between the infrared with larger wavelength and the ultraviolet with shorter wavelength. This wavelength has a frequency range of roughly 430–750 terahertz (THz).

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 The primary properties of visible light are intensity, propagation-direction, frequency or wavelength spectrum and polarization. Its speed in a vacuum is 299 792 458 metres a second (m/s), is one of the fundamental constants of nature, same as all electromagnetic radiation (EMR).  Light is found to always move at this speed in a vacuum.

In physics, the term 'light', whether visible or not, sometimes refers to electromagnetic radiation of any wavelength. In this context, gamma rays, X-rays, microwaves, and radio waves are also light.

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 Like all types of electromagnetic radiation, visible light propagates as waves. The energy imparted by the waves is absorbed at single locations, the way particles are absorbed. The absorbed energy of the electromagnetic waves is called a photon and represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave instantly collapses to a single location and this location is where the photon "arrives". This is what is called the wave function collapse. This dual wave-like and particle-like nature of light is known as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

The main source of light for humans on earth has been the Sun, the Moon, fire, radioactive materials etc.

Electromagnetic spectrum and visible light

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The electromagnetic radiation (EMR) is classified into radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays. The behaviour of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.

EMR in the visible light region consists of quanta (called photons) that are at the lower end of the energies that can cause electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina, which change triggers the sensation of vision.

There exist animals that are sensitive to various types of infrareds, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it.

Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the very short (below 360 nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses (such as insects and shrimp) are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm under ideal laboratory conditions, people can see infrared up to at least 1,050 nm; children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm.


The study of light and the interaction of light and matter is termed optics.


Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by Snell's Law:

N1.sin θ1 = N2.sin θ2

where θ1 is the angle between the ray and the surface normal in the first medium, θ2 is the angle between the ray and the surface normal in the second medium and N1 and N2 are the indices of refraction, N = 1 in a vacuum and N > 1 in a transparent substance.


When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.


The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.


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Saturn is the sixth planet from the Sun and the second-largest planet in our solar system.

It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. Saturn is named after the Roman god of wealth and agriculture.